The present disclosure relates generally to magnetic resonance imaging, and particularly to a magnetic field generating apparatus for magnetic resonance imaging.
High field MRI magnets typically use sets of superconducting coils placed within a liquid helium vessel, which is typically referred to as a 4K (4-degree Kelvin) envelope. The coils produce a homogeneous magnetic field within a required field of view (FOV) by balancing harmonics generated by each coil. The balance is achieved by the appropriate selection of coil ampere-turns, strategic placements, and aspect ratios. The limit of the achievable level of homogeneity (that is, maximum size of FOV) is dictated by the allowable coil envelope and accepted design criteria, such as conductor volume, peak field Bpeak, mechanical stresses, and other criteria which correlate with the cost and robustness of the design. Besides coils, other field generating means, such as magnetized steel rings may be employed in the design, which may help in shaping the homogeneous FOV.
The homogeneity of a magnetic field may be determined by evaluating the coefficients of a Legendre polynomial expansion that describes the magnetic field. Each coefficient represents a different harmonic order, axial harmonics being referred to as Zn, where n is the order of the harmonic, for example n=2 refers to the second order harmonic. The value of each Zn term is an indication of the field homogeneity at a specified spherical volume, with lower absolute values indicating higher field homogeneity. Amplitude of each harmonic created by a magnetized ring, or a coil, depends on the ring (or coil) location and radius; the smaller the radius, the higher the relative contribution of high order harmonics is in the total mix created by the ring (or coil). If field shaping steel rings were disposed within the helium vessel, they would be relatively far from the FOV and therefore generate a mix of harmonics where low orders dominate. However, homogeneity in the axial (z) dimension of the field of view is mostly defined by the high order harmonics, which in a typical whole body MRI imager, often starts with the eighth order and higher.
Demand for a wider-angle MRI with a challenging coil envelope (short length and large bore) to enhance comfort for claustrophobic and bariatric patients comes in conflict with a full-size field of view, especially in the axial (z) direction, which creates a trade-off between the magnet length and the axial field of view (FOVz) dimension. Accordingly, there is a need in the art for an extended axial field of view arrangement that overcomes these drawbacks.